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Starts With A Bang

Ask Ethan: How Would You Explain The Big Bang To A Child?

It’s something most adults don’t understand very well. So what should you tell a child?


If you’ve ever had a conversation with an inquisitive, curious child, you might have experienced that they all end the same way. They’ll begin by asking where something comes from or how something works, a behavior you very much want to encourage. But then, when you answer that, there’s the inevitable follow-up. Your answer now becomes the topic of a new question, which unfolds into a conversation that eventually runs into the limits of your (or even humanity’s) knowledge. At some point, you may even run into questions about the very beginning of it all: the Big Bang. That’s where this week’s question comes from, courtesy of Tyler Legare, who asks:

How would you explain the big bang to a 10-year-old?

Even though the Big Bang is something that most adults don’t fully understand, it’s a story that science knows the answer to. Here’s how I’d tell it to a 10-year-old.

The human body, as we conventionally think of it, is composed of organs that are made of cells. But at an even smaller level, everything within us is composed of atoms: an enormous number of them due to their overwhelmingly tiny size. (PIXABAY USER PUBLICDOMAINPICTURES)

So, you want to know where it all comes from? Everything, from you and me here on Earth to all the planets, stars, and galaxies in the Universe? Well, so did pretty much every curious person who’s ever lived. And for most of human history — for thousands upon thousands of years — we only had stories, guesses, and speculations. What we didn’t have until very recently, over the last 100 years or so, was a scientific answer.

That answer is a term you might have heard before: the Big Bang. The Big Bang is where everything that we have in our Universe today came from. It’s the secret to understanding how our Universe got to be the way it is today, and the key to unlocking the ancient history of what our Universe was like long ago. To get a feeling for how important this is, let’s take a look at what we actually see when we look at the Universe today.

The sizes of composite and elementary particles, with possibly smaller ones lying inside what’s known. With the advent of the LHC, we now can restrict the minimum size of quarks and electrons to 10^-19 meters, but we don’t know how far down they truly go, and whether they’re point-like, finite in size, or actually composite particles. (FERMILAB)

When we look around at everything on Earth, there are all sorts of things to see, hear, smell, taste, and touch. Everything that our body is capable of interacting with — other people, food, air, even light — is made of matter and energy. This isn’t just true of the things we find on Earth, of course. Wherever we look in the Universe, from other planets to stars to distant galaxies and beyond, we find the same things: matter and energy, made out of the same basic building blocks we find here on Earth.

The only reason we can get such complicated things as human beings out of these basic building blocks is because there are so many possible ways that the fundamental bits of matter and energy can bind together. The iron in our blood, the calcium in our bones, and the sodium in our nerves are just a few examples of how these tiny atomic building blocks can bind together to create something as complex and intricate as our entire bodies.

A portion of the Hubble eXtreme Deep Field in full UV-vis-IR light, the deepest image ever obtained. The different galaxies shown here are at different distances and redshifts, and allow us to understand how the Universe is both expanding today and how that expansion rate has changed over time. (NASA, ESA, H. TEPLITZ AND M. RAFELSKI (IPAC/CALTECH), A. KOEKEMOER (STSCI), R. WINDHORST (ARIZONA STATE UNIVERSITY), AND Z. LEVAY (STSCI))

Beyond our own planet, the Universe is vast, enormous, and full of stuff. There are hundreds of billions of stars in our Milky Way galaxy, and practically every star ought to have its own system of planets. But the Milky Way is just one of perhaps two trillion galaxies present in the Universe we can see. And what’s remarkable about all of them is, with only a few dozen exceptions, they all appear to be moving away from us.

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

This was an enormous surprise when it was first discovered way back in the 1920s. Why should almost every galaxy in the Universe be speeding away from us? And it gets worse: the farther away a galaxy is, the faster it appears to speed away from us.

Why would it be doing this? The answer can be found in a ball of dough filled with raisins.

The ‘raisin bread’ model of the expanding Universe, where relative distances increase as the space (dough) expands. The farther away any two raisin are from one another, the greater the observed redshift will be by time the light is received. The redshift-distance relation predicted by the expanding Universe is borne out in observations, and has been consistent with what’s been known all the way back since the 1920s. (NASA / WMAP SCIENCE TEAM)

If you want to properly bake your dough into raisin bread, you first have to let the bread leaven. That means you mix your dough up, you put your raisins in it, and then you cover it and put it in a warm, dry place to let it rise. Over time, the dough will double in size, but the raisins inside your bread will just remain normal raisins.

But what would you see if you were one of the raisins, and you could only see the other raisins, and not the dough itself? As time went on and the dough continued to rise, every raisin would appear to get farther apart from every other raisin. The farther away they are, the faster they’d appear to move apart.

Well, in our Universe, the raisins are individual galaxies, and the dough is the invisible fabric of space.

There is a large suite of scientific evidence that supports the picture of the expanding Universe and the Big Bang, complete with dark energy. The late-time accelerated expansion doesn’t strictly conserve energy, but the reasoning behind that is fascinating as well. (NASA / GSFC)

If space itself is expanding like this, then that means the Universe is getting bigger and the galaxies are getting farther apart as time goes forward. But that also means, if we wanted to imagine what the Universe was like in the past, that space was smaller. If we only looked at the raisins, that would mean the Universe was denser in the past, with more galaxies (and more matter) in the same amount of space early on, and with less of them later on.

This is the “big idea” of the Big Bang. Things that aren’t held together, like any two well-separated galaxies, are getting farther apart as time goes on. But this also means that they were closer together in the past. And if we extrapolate backwards in time, farther and farther, we can imagine that everything — all the matter and energy we can see — was once concentrated into one super-small region long ago.

How matter (top), radiation (middle), and a cosmological constant (bottom) all evolve with time in an expanding Universe. As the Universe expands, the matter density dilutes, but the radiation also becomes cooler as its wavelengths get stretched to longer, less energetic states. Dark energy’s density, on the other hand, will truly remain constant if it behaves as is currently thought: as a form of energy intrinsic to space itself. (E. SIEGEL / BEYOND THE GALAXY)

The Big Bang is this entire picture of our Universe’s history. Everything that exists today began, billions of years ago, in one small region of space. That space has been expanding ever since, and all the matter and energy that was present back then is still present today. It’s just more spread-out now, driven apart by the expansion of the Universe.

But the Big Bang isn’t just an origin story; it’s the only scientifically valid explanation for how the Universe grew to be the way it is today. To understand how, there’s only one more piece to the puzzle: the fact the pure energy in the Universe — in the form of light, or radiation — gets cooler when the Universe gets larger, and was hotter when the Universe was smaller. The farther back in time we look, we find a Universe that’s not only denser, but also hotter.

This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding Universe. Note that the objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them. (ROB KNOP)

This still means that the earliest stages of the Big Bang still have all the matter that’s in our Universe today. But all that matter is not only compressed into a tiny amount of space, but that space is filled with large amounts of hot radiation. In the earliest stages, you cannot even make different kinds of atomic nuclei: the cores of atoms like iron, calcium, sodium, oxygen, or carbon. Only when the Universe has expanded (and cooled) enough does that happen.

Much later, the Universe expands and cools enough that we can form neutral atoms. All that radiation — which blasted atomic nuclei apart earlier on, and blasted neutral atoms apart for much longer — should still be around today. If the Big Bang were correct, we should be able to go out and look for it. In 1964, scientists finally discovered it, and by today (2020), we’ve measured it exquisitely. It’s real, and it’s definitely what the Big Bang predicted.

Arno Penzias and Bob Wilson at the location of the antenna in Holmdel, New Jersey, where the cosmic microwave background was first identified. Although many sources can produce low-energy radiation backgrounds, the properties of the CMB confirm its cosmic origin. (PHYSICS TODAY COLLECTION/AIP/SPL)

The Universe continued to expand and cool, but it also began to gravitate, where little tiny clumps of matter began attracting other clumps of matter. Over time, they grew together, with the largest clumps overcoming the expansion of the Universe. These lucky winners eventually grew into stars and galaxies, which gave rise to heavy elements, rocky planets, and in at least one case, intelligent life.

The Big Bang taught us how the Universe as-we-know-it began. It taught us how the Universe grew up from this ultra-dense early state all the way up to the present day. It’s a remarkable story, but one that isn’t over yet. The Universe continues to expand today, and that’s something that’s of tremendous interest to scientists. The next great mystery that we’re still trying to solve, however, is how it will all eventually end. Maybe you’ll be the one who finally figures it out.

The different ways dark energy could evolve into the future. Remaining constant or increasing in strength (into a Big Rip) could potentially rejuvenate the Universe, while reversing sign could lead to a Big Crunch. Under either of those two scenarios, time may be cyclical, while if neither comes true, time could either be finite or infinite in duration to the past. (NASA/CXC/M.WEISS)

Send in your Ask Ethan questions to startswithabang at gmail dot com!

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.

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